PREP法制备球形NiTi合金粉末的特性及显微组织

Properties and microstructures of spherical NiTi powders prepared by plasma rotating electrode process

以NiTi合金棒材为原料,采用PREP法(plasma rotating electrode process,等离子旋转电极雾化法)制备球形镍钛(NiTi)合金粉末,粉末粒度主要分布在60~125μm之间。通过扫描电镜(SEM)、X射线衍射(XRD)与差示扫描量热分析(DSC)等,对不同粒度的NiTi粉末显微组织进行表征。结果表明,粒径较大颗粒(≥178μm)的冷凝组织为胞状晶组织,小尺寸颗粒(≤40μm)表面光滑,无明显结晶组织;大尺寸颗粒主要以B2相为主,同时含有少量Ni_3Ti、NiTi_2二次相,随粉末粒度减小,二次相的生长受到抑制;不同粒径的NiTi合金粉末由于冷却速率不同最终导致相变点温度不同,粒度≥178μm的大颗粒粉末由于发生共析反应生成二次相,使得B2-NiTi相内Ni含量减少,最终导致该粉末的相变点温度高于粒度≤40μm粉末的相变点温度。

The spherical NiTi powders were produced by a plasma rotating electrode process （PREP） using NiTi alloy bar as raw materials. The particle size of NiTi powders ranged from 60 to 125 μm. The microstructures and phases of NiTi powders with various particle size were characterized by SEM, XRD and DSC. The results show that larger powders exhibit cellular structure grains, while smaller powders demonstrate smooth surface and non-crystal structure. The larger powders are mainly composed of B2-NiTi phase and a small amount of Ni3Ti and NiTi2 secondary phases. However, with decreasing particle size, the secondary phases are suppressed. NiTi powders with different particle sizes show variation of phase transformation temperature point due to the effect of particle size and cooling rate. The secondary phases are generated in the large powders with particle size ≥ 178 μm due to the eutectoid reaction, which results in the decrease of the content of Ni in B2-NiTi phase, leading to higher phase transition temperature of larger powders than that of the powders with particle size ≤40 μm. Key words： plasma rotating electrode process; spherical powder; NiTi; cooling rate; martensitic transformation